Some plants do cause problems, of course. Poison ivy is a “problem” plant for humans in that it causes lots of people to break out in allergic reactions, sometimes severely.

Yet poison ivy is in fact a widespread native species, providing wildlife food for a variety of critters. It is a common component of many natural ecosystems and has been here in our landscapes a lot longer than we humans have.

On the other hand, there are some plants that have not been around very long, relatively speaking. These are the weeds.

Some weeds are rather innocuous, rarely causing problems. But then other weeds are ravenously destructive, showing up in our lawns, gardens, agricultural fields, just about anywhere.

The thing we should remember about weeds, whether benign or destructive, is that nearly all of them have been introduced by humans into a previously weed-free environment. You might think of these introduced species as “guests,” sometimes invited intentionally, sometimes not. The problem with guests, of course, is that sometimes they stay too long.

And then there is our mystery plant.

It is a gorgeous herb, capable of producing masses of beautiful foliage. The sheathing, pointed leaves are produced on flimsy, fleshy stems that flop over and make new roots. I’ve seen some of these plants producing brightly variegated leaves, striped with yellow.

Flowering occurs in the autumn, until frost. Usually, a single flower will pop up at the end of a stem or in a leaf axil. Each flower is about half an inch wide, with three white or pink petals. Small pods form afterward, containing little seeds.

This is a species that occurs naturally in much of southern Asia, from India to Korea and Japan. Here’s the scary thing: It is now a common component of wetland systems in North America, and is surely present in every Southeastern state. It is also known now as a weed in Oregon and Washington.

For a number of years, botanists knew of it in the South only from the coastal plain, but more recently it has invaded piedmont and mountain settings. It is notorious for rampant growth, crowding out everything around it, and it is definitely invasive. How did it arrive?

One theory is that seeds of this plant were introduced — accidentally — into South Carolina late in the 17th century, contaminating rice seed. According to this idea, this species remained confined to rice fields of South Carolina, and then elsewhere in the Southeast, until relatively recently, and then for whatever reasons, it has taken on a new and more threatening behavior, easily spreading into new habitats.

Waterfowl, which like to eat the seeds, are implicated in this spread. The stems root with such ease that it is no surprise fragments of the plants could easily start up after being transported somewhere else.

Although it isn’t “evil”, this plant guest has gained a nasty reputation. And it doesn’t look like it

wants to leave any time soon.

John Nelson is the curator of the A. C. Moore Herbarium at the University of South Carolina, in the Department of Biological Sciences. As a public service, the herbarium offers free plant identifications. For more information, visit www.herbarium.org, call 803-777-8196 or email nelson@sc.edu.